Inductive proximity sensors are non-contact sensing devices that are used in a wide variety of manufacturing processes to sense the presence of metal objects. Proximity sensors contain a sensing coil, and an electronic sensing circuit, and once connected to electricity; the sensor generates a magnetic field emanating from the active sensing face. As a metal object enters into the range of the proximity sensor, the magnetic field is disrupted, and this disruption to the magnetic field is detected by the sensing circuit of the sensor. Inductive proximity sensors are particularly useful in stamping dies, automated welding processes; automated assembly machines and packaging machinery, where moving metal parts are integral to production.
In order to obtain a repeatable manufacturing process, machines need to verify the placement of objects in close relation to the optimally designed location. For this reason, inductive proximity sensors are particularly useful due to their repeatable, albeit short range. Common inductive proximity sensors have a sensing range of only a few millimeters from the active sensing face. For instance, common 12 mm diameter inductive proximity sensors have a sensing range of only 4 mm from the active sensing face to the edge of the detectable magnetic field. This short range is optimal for repeatable manufacturing processes, since the sensor only sends an output to the logic controller once the metal target is within this short range. However, this short range also makes it common for the active sensing face to be contacted by the metal target, or by moving debris in close relation to the sensing area. As a result, inductive proximity sensors must be replaced often, which causes costly downtime to machinery.
In order to increase the working lifespan of inductive proximity sensors it is common practice to produce an Exterior Housing with an interior diameter only slightly smaller than the outside diameter of the Exterior Housing casing. By using a very large interior diameter, it is possible to use a larger sensor coil, which can produce a larger magnetic field and longer effective sensing range. The longer sensing range could keep the active sensing face at a greater distance to the target material, however, the sensing range is only improved by a few millimeters and contact with the sensing face is still common.
In order to increase the durability of the active sensing face, it is common to use a cap or cover over the active sensing face made of ceramic, plastic, Teflon or other materials. By using a non-metallic cover over the sensing face, the sensing range can be improved, since a metallic sensing face could adversely affect the potential sensing range because of effects to the magnetic field produced by the sensing coil when passing through the metal sensing face. These non-metallic materials can improve sensor longevity because they can withstand impacts from metal objects better than a typical plastic sensor face cover. However, these types represent a two piece construction, where one piece is the metal Exterior Housing, and the other piece the sensing face cover. This produces a weaker structural construction than may be obtained from a one piece construction, and these types can still break easily due to heavy force, or repeated contacts.
In order to improve on the durability of inductive proximity sensors, it has been proposed to recess the active sensing face from the tip of the Exterior Housing in order to reduce contact to the active sensing face or to position the sensing coil at a distance from the Exterior Housing using a protective annulus placed between the sensing coil and the outer edge of the housing. Also the coil could be surrounded by an air gap, or other material having a low magnetic permeability relative to the Exterior Housing. This allows for some protection from contact while preserving the sensing range, however, a structural disadvantage exists compared to an all metal one piece construction of the Exterior Housing.
Accordingly, a need exists to improve upon the Exterior Housing construction of inductive proximity sensors.